During recent years, the number of wet scrubber systems installed to remove gaseous and particulate material from stack gases has increased tremendously. These wet scrubbers are being used to clean effluent from boiler stacks, incinerator stacks, limekilns, foundries, blast furnaces, basic oxygen furnaces (BOF), open hearth units, coke plants, paper mill recovery boilers, pet food manufacturing facilities, electric furnaces (steel and aluminum), smelters, asphalt plants and many others.
One of the most important features of a scrubber system is the contact chamber, the device used to effect transfer of gaseous and/or particulate matter from the gas to the liquid phase. Most wet scrubber systems involve a venturi, a packed bed, an orifice plate, a spray chamber, flooded trays or a turbulent bed. Some systems even use two contact chambers in series, for example, a venturi followed by a spray chamber.
Venturi or orifice plate scrubbers are generally more efficient for particulate removal while packed beds, turbulent beds, flooded trays and spray chambers are usually more efficient for removing gaseous components such as SO.sub.2 and HF.
The present invention is directed specifically to those scrubber systems where scaling and deposition problems due to insoluble calcium sulfate and calcium sulfite are encountered. The type of problem which the present invention minimizes is that which is usually found in gas scrubber systems of utility boilers, incinerators or, for that matter, any furnace or boiler system where high sulfur fuels are used and, in particular, where the scrubber systems utilize a wet lime or limestone process.
The flue gas generated by a utility boiler, for example, contains, in addition to the fly ash pollutant, SO.sub.2 and SO.sub.3 gases. These components must be removed before the flue gas is discharged into the atmosphere. Various systems have been used for this purpose, among which may be mentioned the wet sodium carbonate and the wet lime and limestone scrubber systems. In both of these systems excessive calcium sulfate and calcium sulfite formation can be a problem. With the use of lime and limestone, the formation of the calcium sulfite is a necessary requirement for the scrubber system to be effective.
In these systems, lime or limestone is added to the scrubber in slurry form. The limestone slurry is stored in a recycle tank which supplies the absorber tower pumps. These pumps supply the limestone slurry to the top of the absorber tower. The slurry is allowed to cascade downward as the incoming flue gas moves upward. Ideally, turbulent contact between the slurry and gas should be maximized and efficient SO.sub.2 removal should occur.
Sulfur oxides are absorbed and precipitated as calcium salts which, after being separated from the liquid, are either discarded as solid waste or sold for use as wall board fillers and other recycle products. The water which is saturated with respect to calcium sulfite and sulfate is recycled. Closed-loop operation is desired to avoid the discharge of the high solids content water and to enhance SO.sub.2 absorption. The closed-loop operation increases the scaling problem by increasing the supersaturation inside the lime or limestone scrubber. The scale usually consists of CaSO.sub.4 and CaSO.sub.3.
The obvious objective in any system as described is to obtain the most effective removal of the sulfur dioxide gases at the least amount of either equipment or capital costs and/or chemical costs.
With these objectives in mind, industry is continually investigating, designing and developing techniques and chemical materials in an attempt to achieve better sulfur dioxide removal to assure that the regulatory standards are met or adhered to as closely as possible.